rtl8xxxu: Handle 32 bit mailbox extension regs found on 8723bu/8192eu/8812

Gen1 chips use a 16 bit mailbox extension register, for upto 48 bit
mailbox commands. The newer generation chips use a 32 bit mailbox
extension register instead, for upto 64 bit mailbox commands.

Handle writing the larger mailboxes.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>